1. Field of the Invention
The present invention relates to a method and apparatus for efficiently allocating control channel transmission resources when a packet data channel and a control channel are transmitted in the same transmission period in a Single Carrier-Frequency Division Multiple Access (SC-FDMA) wireless communication system.
2. Description of the Related Art
FIG. 1 illustrates a transmitter in a Localized FDMA (LFDMA) system, which is a type of SC-FDMA system. While the transmitter is configured so as to use Discrete Fourier Transform (DFT) and Inverse Fast Fourier Transform (IFFT) in FIG. 1, any other configuration is available to the transmitter.
Referring to FIG. 1, the use of DFT and IFFT facilitates change of LFDMA system parameters with minimal hardware complexity. Concerning the difference between Orthogonal Frequency Division Multiplexing (OFDM) and SC-FDMA in terms of transmitter configuration, the LFDMA transmitter further includes a DFT precoder 101 at the front end of an IFFT processor 102 that is used for multi-carrier transmission in an OFDM transmitter. In FIG. 1, Transmission (TX) modulated symbols 103 are provided in blocks to the DFT precoder 101. DFT outputs are mapped to IFFT inputs in a band including successive subcarriers. A mapper 104 functions to map the transmission modulated symbols to an actual frequency band.
FIG. 2 illustrates a data transmission from User Equipments (UEs) in their allocated resources in a conventional SC-FDMA system.
Referring to FIG. 2, one Resource Unit (RU) 201 is defined by one or more subcarriers in frequency and one or more SC-FDMA symbols in time. For data transmission, two RUs indicated by the diagonal lines are allocated to UE1 and three RUs indicated by the dots are allocated to UE2.
The RUs in which UE1 and UE2 transmit data are fixed in time and successive in set frequency bands. This scheme of resource allocation or data transmission selectively allocates frequency resources that offer a good channel status to each UE, to thereby maximize system performance with limited system resources. For example, the blocks with the diagonal lines offer better radio channel characteristics to UE1 than in other frequency bands, whereas the dotted blocks offer better radio channel characteristics to UE2 than in other frequency bands. The selective allocation of resources with a better channel response is called frequency selective resource allocation or frequency selective scheduling. As with uplink data transmission from a UE to a Node B as described above, the frequency selective scheduling applies to downlink data transmission from the Node B to the UE. On the downlink, the RUs marked with diagonal lines and dots represent resources in which the Node B transmits data to UE1 and UE2, respectively.
However, the frequency selective scheduling is not always effective. For a UE that moves fast and thus experiences a fast change in channel status, the frequency selective scheduling is not easy. To be more specific, although a Node B scheduler allocates a frequency band in a relatively good channel status to a UE at a given time, the UE is placed in an already significantly changed channel environment when it receives resource allocation information from the Node B and is to transmit data in the allocated resources. Hence, the selected frequency band does not ensure a relatively good channel status for the UE.
Even in a Voice over Internet Protocol (VoIP)-like service that requires a small amount of frequency resources continuously for data transmission, if the UE reports its channel status for the frequency selective scheduling, signaling overhead can be substantial. In this case, it is more effective to use frequency hopping rather than the frequency selective scheduling.
FIG. 3 illustrates frequency hopping in a conventional FDMA system.
Referring to FIG. 3, frequency resources allocated to a UE for data transmission change in time. The frequency hopping has the effect of randomizing channel quality and interference during data transmission. As data is transmitted in frequency resources that vary in time, the data has different channel characteristics and is interfered by a different UE in a neighbor cell at each time point, thus achieving diversity.
However, the frequency hopping is not viable when RUs hop in independent patterns in the SC-FDMA system as illustrated in FIG. 3. For instance, if RUs 301 and 302 are allocated to different UEs, it does not matter. Yet, if both the RUs 301 and 302 are allocated to a single UE, they hop to the positions of RUs 303 and 304 by frequency hopping at the next transmission point. Since the RUs 303 and 304 are not successive, the UE cannot transmit data in these two RUs.
In this context, to achieve frequency diversity in the SC-FDMA system, mirroring is disclosed to substitute for the frequency hopping, and is illustrated in FIG. 4.
Conventionally, an RU moves symmetrically with respect to the center frequency of a total frequency band available for data transmission. For example, an RU 401 is mirrored to an RU 403 and an RU 402 to an RU 404 at the next transmission time in Cell A. In the same manner, an RU 405 is mirrored to an RU 406 at the next transmission time in Cell B. The mirroring enables successive RUs to successively hop, thereby satisfying the single carrier property during frequency hopping.
A shortcoming with the frequency hopping with frequency diversity is that the hopping pattern is fixed because there is no way to move RUs without mirroring with respect to a center frequency. This means that frequency diversity is achieved to a certain degree but interference randomization is difficult. As an RU hopped to the opposite returns to its original position by mirroring, only one RU hopping pattern is available. Therefore, even when a plurality of cells exists, each cell cannot have a different pattern.
Referring to FIG. 4, if the RU 402 marked with dots is allocated to a UE in Cell A and the RU 405 marked with single-diagonal lines is allocated to a UE in Cell B for a period of time, the UE of Cell A interferes with the UE of Cell B because only one hopping pattern is available in the mirroring scheme. If the UE of Cell B is near Cell A, it causes substantial interference to UEs in Cell A. As a result, the UE of Cell A using RUs marked with dots suffers from reception quality degradation.